The present invention relates to a developing device for an electrophotographic copier, printer, facsimile transceiver or similar image forming apparatus and, more particularly, to a developing device having a developing roller or similar image carrier for transporting a developer to a developing station where the image carrier faces an image carrier, thereby developing a latent image electrostatically formed on the image carrier.
A conventional developing apparatus for the above application has a developing roller or similar developer carrier for carrying a thin layer of developer thereon and facing a photoconductive element or similar image carrier at a developing station. An electric field is developed at the developing station to transfer the developer from the developing roller to the photoconductive element, thereby developing an electrostatic latent image formed on the photoconductive element. In this type of developing device, a threshold value exists regarding the transfer of the developer from the developing roller to the photoconductive element. This brings about a problem that the developed image has poor tonality since the developer scarcely deposits in an image portion whose surface potential is lower than the threshold value, although it deposits in an image portion whose surface potential is higher than the threshold value. To eliminate this problem, an alternating electric field of relatively low frequency may be formed at the developing station, as disclosed in, for example, Japanese Patent Publication No. 1013/1989. However, when such an alternating electric field is simply applied to the developing station, conditioning the electric field for higher tonality lowers the image density while conditioning it for higher image density unwantedly increases the width of lines of an image.
Moreover, when the above-described type of developing device is operated with developer in the form of a non-magnetic toner, the toner forms powder clouds when moved back and forth between the developing roller and the photoconductive element, lowering the image density to a critical extent (see Japanese Patent Publication No. 14706/1990, for example).